This invention relates to repair of an article that includes a protective internal coating on an internal passage of the article. More particularly, it relates to retention of the internal coating while removing a protective coating on an external surface of the article.
Components of power generation apparatus, for example a turbine engine, operating under relatively high temperature, strenuous environmental conditions generally include surface protection or coatings as a protective barrier between an operating fluid and an article substrate material. One example is a turbine section component of a gas turbine engine operating downstream of a combustor generating products of combustion in an oxidizing atmosphere. Components such as turbine blades and vanes generally are made of a high temperature alloy and are air cooled through at least one internal passage. However, it is common practice to include for additional protection on such component at least an external environmental resistant coating or coating system and frequently an environmental resistant coating on the internal passage.
Examples of high temperature environmental resistant coatings include commercial diffusion aluminide type of coatings, well known and widely used in the gas turbine engine art. Such coatings include Al or an alloy including Al applied to a surface to be protected and heated to diffuse at least a portion of the Al into a substrate of an article. U.S. Pat. No. 3,667,985xe2x80x94Levine et al. describes a form of such an aluminide coating commercially available as Codep aluminide coating. Another type of protective coating widely used commercially with gas turbine engine articles is the Mxe2x80x94Crxe2x80x94Al overlay type of coating in which xe2x80x9cMxe2x80x9d is at least one of Fe, Co, and Ni, and in which at least one other alloying element, typically Y, has been included. Still another widely used type of aluminide coating used in the gas turbine engine art is the Ptxe2x80x94Al type of coating in which Al is diffused into a Pt layer first deposited on a substrate. Types of such environmentally resistant aluminide coatings, in addition to being used externally, frequently are deposited on a surface of an internal passage of an article, for example on the surface of an internal air cooling passage of a turbine blade or vane.
In some forms of external high temperature environmental resistant coatings, an aluminide coating is used as a bond coat between a substrate and an outer ceramic base coating sometimes referred to as a thermal barrier coating (TBC). Various ceramic materials have been used in the ceramic layer, for example zirconia stabilized with yttria, and magnesia stabilized with yttria.
Periodically, as a result of operation under such strenuous operating conditions including impact on the component surface by fluid borne particles or objects, it is necessary to repair such a component. As practiced in the gas turbine engine art, some repair has required removal of at least a portion of the external coating to enable repair of the coating and/or repair of underlying portions of the component. For example, such repair practice has included at least one of removal and replacement of at least a portion of the external coating, and replacement of a portion of the substrate, for example by welding, brazing, etc. One form of external coating removal, for the aluminide type as well as for the TBC type of coating, has included exposure of the external coating to a medium of a reducing gas including a halide ion or gas, typically of fluoride. Examples of such removal methods are described in U.S. Pat. No. 5,614,054xe2x80x94Reeves et al. (patented Mar. 25, 1997); and U.S. Pat. No. 5,728,227xe2x80x94Reverman (patented Mar. 17, 1998).
Although removal of at least a portion of an external coating can be necessary in the repair of an article, removal of an internal coating on a surface of an internal passage generally is not required in the repair procedure. Exposure of the internal coating to the medium that will remove the internal coating along with the external coating not only is not necessary, but also such removal, requiring subsequent replacement of the internal coating, adds to the cost of repair of the article.
Forms of the present invention provide a method and apparatus for use in repairing an article that includes on an external surface an external coating at least a portion of which is removed by exposure to a coating removal medium. The article includes an internal passage with a passage opening through the external surface, the passage including an internal passage coating on a passage surface. In a method form, the internal coating is isolated from the coating removal medium by surrounding and isolating the passage opening, and thereby the internal coating, with a masking assembly before the external coating is removed. Such masking assembly includes a masking member substantially inert to the coating removal medium and disposed, for example within a cavity in the masking member, about the passage opening across a gap between the external surface and the masking member. The masking assembly includes a flexible seal disposed across the gap between the masking member and the external surface, thereby isolating the passage opening from the coating removal medium. Then the method of removing at least a portion of the external coating is conducted by exposing the external coating to the coating removal medium, the internal coating being protected and retained on the surface of the internal passage by the masking assembly.